1. Field of the Invention
This invention relates, in general, to electrical relays and, more specifically, to relays with state indicators.
2. Description of the Prior Art
Relays are used in a wide variety of devices, many of which must be serviced in the field. Trouble shooting of such devices is especially difficult when solid state relays are used in place of conventional electromechanical relays. The difficulty lies in the fact that, with solid state relays, the state or condition of the relay contacts cannot visually be seen by the technician. In other words, when a relay is inspected to see if it is energized, it is not readily apparent from a visual inspection that the "contacts" in the relay are in the proper position.
In addition to the problem of not being able to see any physical contacts in a solid state relay, there are also other difficulties associated with solid state relays which make trouble shooting difficult. With solid state relays, a failure of the relay can occur in three different modes, assuming that the relay is designed and constructed to normally switch alternating current (AC) loads. In one mode, the contacts, or circuit path, fails to close when the relay is energized. In another mode, the contacts can be shorted together and fail to open when the relay is de-energized. A third mode of failure occurs when the solid state components of the relay partially fail and produce a half-wave current conducting path. Since solid state relays capable of switching AC current usually contain at least one thyristor, the half-wave failure occurs when the thyristor incurs polarity sensitive damage such that it conducts current in one direction all of the time, regardless of control signals or the load current waveform. This type of failure is difficult to diagnose conclusively with simple current or voltage measurements using the common electrical multimeter or similar devices. Lacking more sophisticated instrumentation, such as an oscilloscope, the technician often replaces many relays unnecessarily.
Conventional relay monitoring and indicating devices, especially for electromechanical relays, usually indicate the status of the control circuit which is connected to energize or de-energize the relay. In the simplest form, this could consist of an incandescent lamp connected in parallel with the relay coil. Such an indicator does not provide any information about the polarity of a failure nor does it indicate the state of the relay contacts. Other types of indicators combine the energization circuit of the relay with the load circuit connected to the relay contacts to provide other information about the operation of the relay.
U.S. Pat. No. 4,479,117, issued on Oct. 23, 1984, discloses such an arrangement where the visual indicator is responsive to both the energization circuit and the status of the relay contacts. In effect, the state indicator system for the relay in this patent includes a lamp which lights when the contacts are energized properly upon the application of an energizing voltage, and a second lamp which lights to indicate a fault when the contacts do not close when the coil is energized. Although the indicators shown in the referenced patent may be useful in some applications, they are not capable of indicating any polarity failure of the relay contacts nor the condition of the relay contacts irrespective of the control voltage applied to the coil of the relay.
Therefore, it is desirable, and it is an object of this invention, to provide relay apparatus which economically provides a visual indication of the effective contact position of a solid state relay irrespective of the control voltage applied to operate the relay. In addition, it is also desirable, and it is another object of this invention, to provide an indication of a partial failure in the switching elements of the solid state relay, and a polarity associated with the failure.